A knife roller of the described type is known, for example, from U.S. Pat. No. 4,537,588. It serves to cut out the window in manufacturing windowed letter envelopes. While the letter envelope blank is transported from the knife roller to a further work station, the knife roller must at first hold the scrap waste piece resulting from the window cut-out for a certain period of time and then release it into a suction funnel. In order to achieve this the knife roller comprises suction openings to which vacuum may be applied through a suction channel.